The overall goal of this project is to understand how endothelin receptor B (Ednrb) regulates gastrointestinal motor patterns through smooth muscle and neuronal pathways. Signaling through endothelin receptor B (Ednrb) influences motility in the esophagus, stomach, small intestine, and colon. Mutation of the gene that encodes for Ednrb can cause Hirschsprung (HSCR) disease where enteric ganglion cells fail to develop in the distal colon. Our preliminary studies indicate that ENS structure, ENS activity, and Ednrb regulation of smooth muscle are altered in the proximal small intestine in a Hirschsprung mouse model, Ednrbtm1 Ywa, even in mice that appear normal and do not develop aganglionic megacolon. We hypothesize that Ednrb mutation alters regulation of motility even in the absence of aganglionic megacolon. Results from this proposal could have potential implications for HSCR patients and for other patient populations that suffer from gastroesophageal reflux, irritable bowel syndrome, and constipation. Three specific aims are proposed: Aim 1 will compare the structure and function of the ENS in the small intestine between homozygote, heterozygote, and wildtype Ednrb mouse littermates. Density of the ENS in the proximal small intestine will be compared between by analyzing 3-dimensional confocal images. Electrical field studies of intestinal smooth muscle layers will evaluate myenteric neuronal activity. Aim 2 will determine if Ednrb mutation affects endothelin regulation of intestinal smooth muscle activity. Preliminary studies show a reduced smooth muscle response to Ednrb signaling with Ednrb mutation. Organ bath experiments will evaluate how Ednrb mutation affects endothelin regulation of smooth muscle. Aim 3 investigate how Ednrb that is expressed by the enteric nervous system and intestinal smooth muscle interact in the regulation of intestinal smooth muscle activity. Preliminary studies indicate that Ednrb on smooth muscle cells and enteric neurons both alter intestinal smooth muscle activity. Neuronal Ednrb regulation of intestinal smooth muscle activity will be investigated by developing a transgenic mouse model to isolate Ednrb expression to the ENS.